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Chapter 24: Problem 12

Write a reasonable mechanism for the 3 -ketosphinganine synthase reaction, remembering that it is a pyridoxal phosphate-dependent reaction.

Short Answer

Expert verified
The 3-ketosphinganine synthase reaction involves the cofactor pyridoxal phosphate (PLP) in several critical steps. First, the PLP is deprotonated to form a resonating carbanion intermediate. This intermediate forms a quinonoid structure which gets protonated to regenerate the PLP in its active form and create an intermediate ketimine via proton transfer. This ketimine is then hydrolyzed to yield the product, 3-ketosphinganine, and redeploy the PLP enzyme for further catalysis.

Step by step solution

01

Understanding the role of PLP

Pyridoxal phosphate (PLP) is an organic compound that serves as a coenzyme in all transaminases and in certain decarboxylases, deaminases, and racemases. Its active aldehyde group forms a Schiff base linkage (internal aldimine) with the ε-amine group of a specific lysine group of the aminotransferase enzyme. This link provides the basis for the enzyme's catalytic action.
02

Deprotonation of PLP Schiff base

The reaction mechanism begins with the deprotonation of the PLP internal aldimine by a base. This active form of the enzyme is able to deprotonate the substituent alpha to the carbonyl in the 3-ketodihydrosphinganine. This will generate a carbanion intermediate which is resonance stabilized by the PLP.
03

Creation of quinonoid intermediate and protonation

The carbanion intermediate from the previous step rearranges to form a quinonoid intermediate. This intermediate then gets protonated to produce the product and the internal aldimine.
04

Formation of ketimine intermediate and hydrolysis

The internal aldimine transfers a proton to the alpha carbon of the substrate, and a ketimine intermediate is formed. The ketimine is then hydrolyzed to yield the product (3-ketosphinganine) and regenerate the PLP enzyme.

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